A hypodermic syringe consists of a cylindrical barrel, most commonly made of thermoplastic material or glass, with a distal end adapted to be connected to a hypodermic needle and a proximal end adapted to receive a stopper and plunger rod assembly. The stopper provides a relatively air-tight seal between itself and the syringe barrel so that movement of the stopper up and down the barrel will cause liquid, blood or other fluids to be drawn into or forced out of the syringe through the distal end. The stopper is moved along the syringe barrel by applying axial force on a rigid plunger rod which is connected to the stopper and is sufficiently long to be accessible outside of the barrel.
Hypodermic needle assemblies, typically including a cannula and a hub, are often removably attached to syringes for performing a variety of tasks such as the delivery of medication into patients and into devices, and for withdrawing fluid samples from patients and from fluid sources. Usually, the hub of the hypodermic needle assembly has tapered interior surface adapted to engage the tapered tip of the syringe barrel so that the two components are joined in a frictional interference fit. The tapered syringe tip and the complementarily tapered receptacle in the hub are referred to as standard luer fittings. A wide variety of other medical devices such as stopcocks and tubing sets have standard luer fittings which allow them to be engaged to a syringe tip.
It is important that the frictional fit between the syringe tip and the needle hub or other medical device is strong enough to prevent accidental disengagement caused by the fluid pressures within the syringe and/or other factors such as forces applied to the needle hub when actuating safety needle shields connected to the hub. If the syringe tip becomes disengaged from the needle assembly, medication, blood or other fluids will be lost, and there is also potential for contamination.
The prior art teaches many structures for improving the connection between medical devices having tapered luer fittings such as needle assemblies and syringes. These structures include complementary engaging structure on both the needle hub and syringe barrel tip such as projections and recess providing for a snap-fit arrangement. Manually releasable locking structures are provided to increase the connection between the needle hub and barrel tip while allowing reasonable forces for disconnections of these components. Also, enhancements to the luer tip of the syringe barrel such as coatings, sandblasting and mechanical collars have provided for improved connection between a needle hub and a syringe barrel tip. Many of the structures taught by the prior art do not contemplate the subsequent removal of the needle assembly from the syringe barrel. Others require extensively modified needle hubs and barrel tips. Structures having a tapered luer fitting such as a needle assembly and syringe barrel are adequate for normal use when the needle assembly is properly installed on the syringe tip. Difficulties can arise if the user does not use enough force to frictionally engage the luer tapered surfaces which can result in inadvertent disconnection of the needle assembly.
Although the prior art teaches various devices and structures for improving the strength of the connection between a syringe barrel and the hub of a needle assembly or other fluid handling device, there is still a need for a simple, straight-forward, reliable needle hub or other fluid-handling device having structure which improves the strength of the connection with the syringe tip or other device having a standard tapered luer tip. There is also a need for a needle hub requiring consistent forces for disengagement even when less than the desired force is used for engagement.